Isoprene is an expensive monomer to manufacture, but highly desirable as a synthetic rubber feedstock. Not only are the polyisoprenes satisfactory for much the same uses as natural rubber, the polyisoprenes for many purposes actually are superior, being pure in composition.
Isoprene presently is manufactured, or recovered from various refinery streams such as naphtha cracker by-product streams, only in relatively limited quantities. Small amounts of isoprene are produced in ethylene plants, or can be recovered from by-product cracked gasoline produced by some ethylene plants. Some isoamylenes, precursors of isoprene, can be recovered from some cat cracker effluents and dehydrogenated to isoprene. Efforts have been made to produce isoprene by a by-product route involving propylene dimerization, but this has proven uneconomical.
Another source of feedstock that potentially could be useful in making isoamylenes by the triolefin disproportionation reaction has been the availability of various mixed butenes streams from such as a naphtha cracker C.sub.4 stream of butylenes, or catalytic cracker effluent cut of butylenes. The stumbling block in any such approach has been the need to provide a balanced feed to the triolefin reaction of butenes-2 and isobutene without butene-1. Heretofore, efforts have been made in treating such mixed butenes streams to separate and purify each component, and then recombine butenes-2 and isobutene streams in a suitable proportion for disproportionation. These efforts have been costly, and prevented development of a suitable technology.